Laterally diffused metal oxide semiconductor transistor with partially unsilicided source/drain

ABSTRACT

A method of fabricating a laterally diffused metal oxide semiconductor (LDMOS) transistor includes forming a dummy gate over a substrate. A source and a drain are formed over the substrate on opposite sides of the dummy gate. A first silicide is formed on the source. A second silicide is formed on the drain so that an unsilicided region of at least one of the drain or the source is adjacent to the dummy gate. The unsilicided region of the drain provides a resistive region capable of sustaining a voltage load suitable for a high voltage LDMOS application. A replacement gate process is performed on the dummy gate to form a gate.

TECHNICAL FIELD

The present disclosure relates generally to an integrated circuitdevice, more particularly, to a laterally diffused metal oxidesemiconductor (LDMOS) transistor.

BACKGROUND

LDMOS transistors are used in radio frequency (RF)/microwaveapplications. For example, in power amplifier applications, high outputpower is required. Therefore, an LDMOS transistor that can handle a highvoltage and an increased current is desirable. Also, an LDMOS transistorthat uses a polysilicon/SiON gate stack has a control issue in thesilicide formation on its gate, resulting in a partially silicided gateduring the silicide process. Accordingly, new methods and structures forLDMOS transistors are desired.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of embodiments of the presentinvention, and the advantages thereof, reference is now made to thefollowing descriptions taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 illustrates a laterally diffused metal oxide semiconductor(LDMOS) transistor in accordance with one or more embodiments;

FIG. 2-FIG. 7 illustrate various stages of an exemplary method offabrication of the LDMOS transistor in accordance with one or moreembodiments.

DETAILED DESCRIPTION

The making and using of various embodiments are discussed in detailbelow. It should be appreciated, however, that the present disclosureprovides many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use, and do notlimit the scope of the invention.

A structure for a laterally diffused metal oxide semiconductor (LDMOS)transistor is provided. FIG. 1 illustrates a laterally diffused metaloxide semiconductor (LDMOS) transistor in accordance with one or moreembodiments. The LDMOS transistor 100 includes a substrate 102, a metalgate 111 over the substrate 102, and a source 104 and a drain 108 overthe substrate 102 on opposite sides of the metal gate 111. The substrate102 may comprise a bulk silicon or silicon-on-insulator substructure.Alternatively, the substrate 102 may comprise other materials, which mayor may not be combined with silicon, such as germanium, indiumantimonide, lead telluride, indium arsenide, indium phosphide, galliumarsenide, gallium antimonide, or any other suitable material. A gatedielectric layer 116 is disposed over the substrate 102 and between themetal gate 111 and a channel 122 of the transistor 100. Spacers 118 arealso shown on opposite sides of the metal gate 111.

A first silicide 106 is disposed on the source 104. A second silicide110 is disposed on the drain 108. A silicide is an alloy of silicon andmetals, used as contact materials in silicon device manufacturing, e.g.,TiSi₂, CoSi₂, NiSi, other silicide, or any combinations thereof. Thesilicide combines advantageous features of metal contacts (e.g.,significantly lower resistivity than polysilicon) and polysiliconcontacts (e.g., no electromigration). The drain 108 has an unsilicidedregion 120 adjacent to the metal gate 111 to provide a resistive regioncapable of sustaining a voltage load suitable for a high voltage LDMOSapplication.

For example, LDMOS transistors are widely used in power amplifiers forbase-stations that use a high output power with a corresponding drain tosource breakdown voltage usually above 60 volts. Many high power RFapplications use direct current (DC) supply voltages ranging from about20 to about 50 volts.

In one example, the unsilicided region 120 can be defined between thespacer 118 and the silicide 110. In some embodiments, the unsilicidedregion 120 has a length ranging from about 0.05 μm to about 1 μm. Thegate dielectric layer 116 comprises a high-k gate dielectric material.The materials that may be used to make the high-k gate dielectricinclude hafnium oxide, hafnium silicon oxide, lanthanum oxide, zirconiumoxide, zirconium silicon oxide, tantalum oxide, titanium oxide, bariumstrontium titanium oxide, barium titanium oxide, strontium titaniumoxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, leadzinc niobate, or any other suitable material. The metal gate 111 cancomprise more than one metal layers, e.g. a work function metal layer114 and a trench fill metal layer 112. The trench fill metal layer 112can comprise tungsten, aluminum, titanium, titanium nitride, or anyother suitable material.

Table 1 below shows a performance comparison between a LDMOS transistorusing a polysilicon (“poly”)/SiON gate stack and a LDMOS transistorusing a high-k dielectric/metal gate stack in accordance with one ormore embodiments. The LDMOS transistors have the same equivalent oxidethickness (EOT) of 28 angstroms and the gate length Lg of 1 μm. Theon-state current I_(on) of the LDMOS transistor using a high-kdielectric/metal gate stack in accordance with one or more embodimentsshows about 4% increase compared to the on-state current I_(on) of theLDMOS transistor using the poly/SiON gate stack, due to the eliminationof the poly depletion effect.

TABLE 1 LDMOS Poly/SiON HK/MG V_(t lin) (V) 0.256 0.257 V_(t sat) (V)0.251 0.251 I_(d lin) (μA/μm) 19.6 20.1 I_(on) (μA/μm) 402.5 419.9 DIBL(mV) 5.4 5.3

The current I_(d lin) in the linear region of the LDMOS transistor usingthe high-k dielectric/metal gate stack in accordance with one or moreembodiments also has an increased value. The threshold voltagesV_(t lin) in the linear region and V_(t sat) in the saturation regionfor both the LDMOS transistors are similar to each other. The draininduced barrier lowering (DIBL) values are also similar to each other.

Regarding on-state resistance distribution, an exemplary embodiment ofthe LDMOS transistor using the high-k dielectric/metal gate stack showeda source resistance of 107 ohm-cm, which is about 4.3% of the totalon-state resistance of the transistor. The channel resistance was 1460.2ohm-cm, which is about 58.7% of the total on-state resistance of thetransistor. The drain resistance was 920.4 ohm-cm, which is about 37% ofthe total on-state resistance of the transistor. These values aresimilar to the on-state resistance distribution of a LDMOS transistorusing the poly/SiON gate stack.

FIG. 2-FIG. 7 illustrate various stages of an exemplary method offabrication of the LDMOS transistor in accordance with one or moreembodiments. FIG. 2 shows a dummy gate 202 formed over the substrate102. In one example, the dummy gate 202 can comprise, for example,polysilicon. A hardmask 204 is formed over the dummy gate 202. The hardmask 204 and the dummy gate 202 may be patterned by applying aphotolithographic process and/or an etch process. In one example, thepolysilicon dummy gate 202 has a thickness between about 100 and about2,000 angstroms. The hardmask 204 may comprise silicon nitride, silicondioxide and/or silicon oxynitride, and may have a thickness betweenabout 100 and about 500 angstroms.

The spacers 118 are formed on opposite sides of the dummy gate 202. Thespacers 118 may comprise, for example, silicon nitride. After formingthe spacers 118, the source 104 and the drain 108 are formed over thesubstrate 102 on opposite sides of the dummy gate 202. The source 104and the drain 108 may be formed by implanting ions into the substrate102, followed by applying an appropriate anneal step. After the ionimplantation and anneal steps, portions of the source 104 and the drain108 may be converted to silicides as described below.

In FIG. 3, a resist protect oxide (RPO) layer 302 is formed over thehardmask 204, the dummy gate 202, the spacers 118, the source 104, andthe drain 108. After covering all device area defined over the surfaceof the substrate 102 with the RPO layer 302, the device area can bedivided into an area that is to be silicided for electrical contacts andanother area that is not to be silicided. In one example, the RPO layer302 is formed using silicon dioxide.

In FIG. 4, the RPO layer 302 (shown in FIG. 3) is partially etched away,leaving the RPO layer 402 over at least a portion of the dummy gate 202,extending over the drain 108. The device area that is not to besilicided is covered with the RPO layer 402. The RPO layer 402 can bedefined by applying, for example, an oxide wet etch that partiallyremoves the RPO layer 302. This protects the areas under the RPO layer402 from the silicide formation. The hardmask 204 also protects thedummy gate 202 from the silicide formation.

In FIG. 5, a silicide process is performed to form the first silicide106 and the second silicide 110. The first silicide 106 is formed on thesource 104. The second silicide 110 is formed on the drain 108 so thatthe unsilicided region 120 of the drain 108 is left adjacent to thedummy gate 202. The unsilicided region 120 of the drain 108 provides ahighly resistive region capable of sustaining a voltage load suitablefor a high voltage LDMOS application as described above. In one example,the unsilicided region 120 has a length ranging from about 0.05 μm toabout 1 μm.

In FIG. 6-FIG. 7, a replacement gate process is performed on the dummygate 202 (shown in FIG. 5). In FIG. 6, after forming the silicidedregions 106 and 110 over the source 104 and the drain 108, the RPO layer402 (shown in FIG. 5) can be removed and a dielectric layer 602 can bedeposited over the device area. The dielectric layer 602 may comprisesilicon dioxide, a low-k material, or any other suitable material. Thedielectric layer 602 may be doped with phosphorus, boron, or otherelements, and may be formed using a high-density plasma depositionprocess.

FIG. 6 shows the dielectric layer 602 after being polished for thereplacement gate process. A chemical mechanical polishing (CMP)operation may be applied to remove a part of the dielectric layer 602.After exposing the hardmask 204 (shown in FIG. 5), the hardmask 204 isremoved to expose the dummy gate 202. In some embodiments, the hardmask204 may be polished from the surface of the dummy gate 202 whendielectric layer 602 is polished. The dummy gate 202 (shown in FIG. 5)that is bracketed by the spacers 118 is removed to create a trench 604between the spacers 118. A selective wet etch process can be applied toremove the dummy gate 202.

In FIG. 7, the gate dielectric layer 116 is formed over substrate 102 atthe bottom of trench 604 after removing the dummy gate 202. The trench604 is filled with the metal gate 111 over the gate dielectric layer116. Although the gate dielectric layer 116 may comprise any materialthat may serve as a gate dielectric for a transistor that includes ametal gate electrode, the gate dielectric layer 116 can comprise ahigh-k dielectric material.

The gate dielectric layer 116 may be formed over the substrate 102 usinga deposition method, e.g., a chemical vapor deposition (CVD), a lowpressure CVD, or a physical vapor deposition (PVD) process. In manyapplications, a high-k gate dielectric layer can be less than about 60angstroms in thickness. In some embodiments, to remove impurities fromthe gate dielectric layer 116 and to increase that layer's oxygencontent, a wet chemical treatment may be applied to the gate dielectriclayer 116.

In some embodiments, the metal gate 111 can comprise more than one metallayers. For example, the work function metal layer 114 can be depositedover the gate dielectric layer 116, and the trench fill metal layer 112can be deposited over the work function metal layer 114. The workfunction metal layer 114 for NMOS transistors can comprise hafnium,zirconium, titanium, tantalum, aluminum, their alloys, e.g., metalcarbides that include these elements, i.e., hafnium carbide, zirconiumcarbide, titanium carbide, tantalum carbide, and aluminum carbide, orany other suitable material. The work function metal layer 114 may beformed on the gate dielectric layer 116 using PVD and/or CVD processes,e.g., sputter and/or atomic layer CVD processes.

In some embodiments, the work function metal layer 114 for NMOStransistors can have a work function that is between about 3.9 eV andabout 4.2 eV. The work function metal layer 114 can fill up the trenchand have a thickness that is between about 100 angstroms and about 2,000angstroms if no trench fill metal layer 112 is deposited. If the trenchmetal 112 is deposited over the work function metal 114 to fill thetrench 604, the trench fill metal layer 112 can comprise a material thatmay be easily polished, e.g., tungsten, aluminum, titanium, titaniumnitride, and/or any other suitable material. In such an embodiment, thework function metal layer 114 may have a thickness between about 50 andabout 1,000 angstroms.

The work function metal layer 114 for PMOS transistors can compriseruthenium, palladium, platinum, cobalt, nickel, conductive metal oxides,e.g., ruthenium oxide, or any other suitable material. In someembodiments, the work function metal layer 114 for PMOS transistors canhave a work function that is between about 4.9 eV and about 5.2 eV.

In the embodiments specifically disclosed with respect to FIGS. 1-7, thedrain 108 outside the spacers 118 is partially unsilicided. However, insome other embodiments, the source 104 outside the spacers 118 ispartially unsilicided, whereas the drain 108 outside the spacers 118 isfully silicided. In further embodiment, both the source 104 and thedrain 108 outside the spacers 118 are partially unsilicided. Asubstantially same effect is achieved in that the unsilicided region(s)of the drain 108 and/or the source 104 provide(s) one or more highlyresistive regions capable of sustaining a voltage load suitable for ahigh voltage LDMOS application as described above.

Embodiments of the fabrication process described above may provideimproved process control of the gate region compared to a known processfor a poly/SiON gate stack, because no silicide formation is necessaryon the gate region. Also, the high-k dielectric/metal gate stack enablesthe structure to have less process variations. To reduce process costand complexity, the LDMOS can be fabricated through a careful logicoperation to define silicide area and manipulation of existingfabrication processes. A skilled person in the art will appreciate thatthere can be many embodiment variations of this invention.

Although embodiments of the present invention and their advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, and composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the present invention,processes, machines, manufacture, compositions of matter, means,methods, or steps, presently existing or later to be developed, thatperform substantially the same function and/or achieve substantially thesame result as the corresponding embodiments described herein may beutilized according to the present invention. Accordingly, the appendedclaims are intended to include within their scope such processes,machines, manufacture, compositions of matter, means, methods, or steps.

1. A method of fabricating a laterally diffused metal oxidesemiconductor (LDMOS) transistor, the method comprising: forming a dummygate over a substrate; forming a source and a drain over the substrateon opposite sides of the dummy gate; forming a first silicide on thesource and a second silicide on the drain, while leaving an unsilicidedregion of at least one of the drain or the source adjacent to the dummygate to provide a resistive region capable of sustaining a voltage loadsuitable for a high voltage LDMOS application; and performing areplacement gate process on the dummy gate to form a gate.
 2. The methodof claim 1, further comprising forming a hardmask over the dummy gate.3. The method of claim 1, further comprising forming a resist protectoxide (RPO) layer over at least a portion of the dummy gate and over theunsilicided region to protect areas under the RPO layer from a silicideprocess for forming the first and second silicides.
 4. The method ofclaim 1, further comprising forming a first spacer and a second spaceron opposite sides of the dummy gate, wherein the unsilicided region isnot covered by any of the first and second spacers.
 5. The method ofclaim 1, wherein the unsilicided region has a length ranging from about0.05 μm to about 1 μm.
 6. The method of claim 1, wherein the gatecomprises more than one metal layers.
 7. The method of claim 1, whereinthe replacement gate process comprises forming a gate dielectric layerbetween the gate and the substrate.
 8. The method of claim 7, whereinthe gate dielectric layer comprises a high-k dielectric material.
 9. Themethod of claim 7, wherein the replacement gate process furthercomprises depositing a work function metal over the gate dielectriclayer.
 10. The method of claim 9, wherein the replacement gate processfurther comprising depositing a trench fill conductive material over thework function metal, the trench fill conductive material being selectedfrom the group consisting of tungsten, aluminum, titanium, and titaniumnitride.
 11. A laterally diffused metal oxide semiconductor (LDMOS)transistor, comprising: a substrate; a gate over the substrate; a sourceand a drain over the substrate on opposite sides of the gate; a firstsilicide on the source; and a second silicide on the drain, wherein atleast one of the drain or the source has an unsilicided region adjacentto the gate to provide a resistive region capable of sustaining avoltage load suitable for a high voltage LDMOS application.
 12. Thetransistor of claim 11, wherein the unsilicided region has a lengthranging from about 0.05 μm to about 1 μm.
 13. The transistor of claim11, further comprising a high-k gate dielectric layer between the gateand the substrate.
 14. The transistor of claim 11, wherein the gatecomprises more than one metal layers.
 15. The transistor of claim 11,wherein the gate comprises a work function metal.
 16. The transistor ofclaim 15, wherein the gate further comprises a trench fill conductivematerial selected from the group consisting of tungsten, aluminum,titanium, and titanium nitride.
 17. A method of fabricating a laterallydiffused metal oxide semiconductor (LDMOS) transistor, the methodcomprising: forming a dummy gate over a substrate; forming a hardmaskover the dummy gate; forming a first spacer and a second spacer onopposite sides of the dummy gate; forming a source and a drain over thesubstrate on opposite sides of the dummy gate; forming a first silicideon the source adjacent to the first spacer; forming a second silicide onthe drain adjacent to the second spacer while leaving an unsilicidedregion of at least one of the drain or the source between the secondspacer and the second silicide; and performing a replacement gateprocess on the dummy gate to form a high-k gate dielectric layer and agate.
 18. The method of claim 17, further comprising forming a resistprotect oxide (RPO) layer over at least a portion of the dummy gate andover the unsilicided region to protect areas under the RPO layer from asilicide process for forming the first and second silicides.
 19. Themethod of claim 17, wherein the unsilicided region has a length rangingfrom about 0.05 μm to about 1 μm.
 20. The method of claim 17, whereinthe replacement gate process comprises depositing a work function metalas at least a part of the gate.